It is common practice to form a seal between the wall of an opening and a shaft or other cylindrical member extending through the opening by positioning a packing member between the wall of the opening and the outer surface of the member. Packing members are typically annular bodies having a generally V-shaped or truncated V-shaped cross-section. Packing members are, in many cases, formed from an elastomeric material such as rubber or plastic. Such packing members are disclosed in U.S. Pat. No. 5,509,670 issued Apr. 23, 1996, and 5,704,615, issued Jan. 6, 1998, the disclosures of which are incorporated by reference herein for all purposes.
The operation of a device such as a hydraulic cylinder typically involves both static and dynamic forces generated by the friction between the packing member and the cylinder shaft. Static friction must be overcome during the initial actuation of the cylinder whereas dynamic friction is encountered during continued movement of the shaft. The amount of friction between the shaft and the packing member is determined by a number of factors including the amount of surface area of the packing member in contact with the shaft. Preferably, the amount of friction between the shaft and the packing member is minimized while maintaining an effective seal. The amount of force required to overcome static friction and initially move the shaft is typically greater than the force required to overcome dynamic friction after the shaft has started moving. Since the amount of force required to overcome static friction is greater than the amount of force needed to overcome dynamic friction, static friction is an important consideration in the selection and design of packing members. Moreover, during the operation of a hydraulic cylinder, a large difference between the force required to overcome static friction and the force required to overcome dynamic friction may result in the cylinder jerking or "stuttering" during operation. Large hydraulic and mechanical stresses can be generated when a hydraulic cylinder jerks in such a manner, stresses that may damage or cause excessive wear of system components, for example, hydraulic valves, fittings and the hydraulic pump supplying pressurized fluid to the cylinder. Consequently, it is desirable not only that the friction between a packing member and a slidable member passing through the packing member be minimized, but also that the difference in the amount of force required to overcome static versus dynamic friction be minimized.
It is also important that a packing member or seal be formed from a readily available, economically feasible material. One such material is urethane. Some types of urethane have many desirable properties including increased resistance to abrasion and wear as compared to other plastics and elastomers. Urethane is also generally more resistance to cuts and tears than other plastics or elastomers, tends to non-brittle at higher hardness, resisting fracture during shock and impact loading, and maintains dimensional stability over a wide range of temperatures. One drawback to the use of urethane to form packing members or hydraulic seals is the tendency to stick during operation. Thus, it is desirable to reduce the amount of contact between the seal and the cylinder. However, adequate support must also be provided to the sealing edge of the packing or seal, and reducing the size of the contact area alone does not completely address the problem.